Vent systems are used with various types of exhaust producing appliances. Generally, vent systems operate as negative draft systems or as positive pressure systems.
Vent systems are divided into various categories. A Category I vent system operates with a negative draft, and the products of combustion are hot enough to stay well above dew point. Since a major portion of gas flue products is water vapor, condensing can occur when the flue products are allowed to cool below dew point. This can cause corrosion if the wrong materials are used in the vent system. A Category I vent system is typically vented with Type “B” vent materials.
A Category II vent system also operates with a negative pressure in the vent, but the temperature of the flue products has cooled to a point where they may condense into a liquid in the flue. A Category II vent requires vent materials that are resistant to the corrosive effects of flue gas condensate.
Category III and IV vent systems operate with a positive pressure in the vent system. Positive pressure vents must be fully sealed to prevent spillage of flue products into an occupied portion of the building. A Category III vent system maintains flue gas temperatures above dew point and should not condense in the flue. A Category IV vent system's flue products cool below their dew point, therefore it is possible they may condense in the flue.
Material selection and sizing for the vent system of a Category II, III or IV boiler is determined by the information contained in the manufacturer's installation instructions, which is based on performance testing under the requirements of the ANSI standard.
All vent systems other than Category I generally use heat and corrosion resistant materials, which are typically more expensive. Category IV vent materials are typically fully sealed and constructed from heat and corrosion resistant stainless steel to withstand the possible corrosive effects from acidic flue gas condensate.